Power boosters are well known in the art and are used in applications where human input force must be amplified or boosted to act upon a device that requires a force beyond the normal human ability to provide it. One example is in the operation of a clutch in a large commercial truck. N%le it may be possible for certain drivers to exert sufficient force to disengage the clutch once or twice, typically, the clutch force is high enough that the repeated application of such force, as is required in regular driving of the truck, is a burden that is not easily borne by the driver. Also, the requirement of high force to operate the clutch would limit the available pool of truck drivers. Through use of a power booster the input force to the booster can be kept at a more comfortable and attainable range while the amplified output force from the power booster is used to operate the clutch.
A similar concept is shown in U.S. Pat. No. 3,113,489 to Cruse et al. Cruse et al. provide a servomotor that acts to amplify an input force. However, the arrangement of the valving in the Cruse et al. servomotor is such that the pressurized air used to act on the main piston also acts on the valve member that must be moved to allow such air to reach the main piston. The Cruse et al. arrangement requires that, in order to actuate the valve to begin airflow to the main piston, the force of the air must first be overcome. This produces a relatively high initial bias force that must be overcome before the power booster begins to act.
It is, therefore, an object of the present invention to provide a power booster that amplifies mechanical input force in which the amplification factor is not sensitive to the position of the piston.
It is also an object of the invention to provide a power booster that has a minimum bias force that must be initially overcome to initiate booster action.
It is another object of the invention that the power booster include a mechanical linkage that can transfer force between the input and output of the booster, even in the event of a failure in supply of pressurized fluid, so that the power booster is operable, albeit without force amplification.